The present invention relates to a cathode ray tube and, in particular, to a cathode ray tube including one or more deflection aiding electrostatic fields.
Conventional cathode ray tubes (CRTs) are widely utilized, for example, in television and computer displays. One or more electron guns positioned in a neck of a funnel-shaped glass bulb of a CRT direct a corresponding number of beams of electrons toward a glass faceplate biased at a high positive potential, e.g., 30 kilovolts (kV). The faceplate usually has a substantially rectangular shape and is generally planar or slightly curved. Together, the glass bulb and faceplate form a sealed enclosure that is evacuated. The electron gun(s) are positioned along an axis that extends through the center of the faceplate and is perpendicular thereto.
The electron beam(s) is (are) raster scanned across the faceplate so as to impinge upon a coating or pattern of phosphors on the faceplate that produces light responsive to the intensity of the electron beam, thereby to produce an image thereon. The raster scan is obtained by a deflection yoke including a plurality of electrical coils positioned on the exterior of the funnel-shaped CRT near the neck thereof. Electrical currents driven in first coils of the deflection yoke produce magnetic fields that cause the electron beam(s) to deflect or scan from side to side (i.e. horizontal scan) and currents driven in second coils of the deflection yoke produce magnetic fields that cause the electron beam(s) to scan from top to bottom (i.e. vertical scan). The magnetic deflection forces typically act on the electrons of the beam(s) only in the first few centimeters of their travel immediately after exiting the electron gun(s), and the electrons travel in a straight line trajectory thereafter, i.e through a substantially field-free drift region. Conventionally, the horizontal scan produces hundreds of horizontal lines in the time of each vertical scan to produce the raster-scanned image.
The depth of a CRT, i.e. the distance between the faceplate and the rear of the neck, is determined by the maximum angle over which the deflection yoke can bend or deflect the electron beam(s) and the length of the neck extending rearward to contain the electron gun. Greater deflection angles provide reduced CRT depth.
Modern magnetically-deflected CRTs typically obtain a xc2x155xc2x0 deflection angle, which is referred to as 110xc2x0 deflection. However, such 110xc2x0 CRTs for screen diagonal sizes of about 62 cm (about 25 inches) or more are so deep that they are almost always provided in a cabinet that either requires a special stand or must be placed on a floor. For example, a 110xc2x0 CRT having a faceplate with an about 100 cm (about 40 inch) diagonal measurement and a 16:9 aspect ratio, is about 60-65 cm (about 24-26 inches) deep. Practical considerations of increasing power dissipation producing greater temperature rise in the magnetic deflection yoke and its drive circuits and of the higher cost of a larger, heavier, higher-power yoke and drive circuitry make increasing the maximum deflection angle so as to decrease the depth of the CRT is disadvantageous.
A further problem in increasing the deflection angle of conventional CRTs is that the landing angle of the electron beam on the shadow mask decreases as deflection angle is increased. Because the shadow mask is as thin as is technically reasonable at an affordable cost, the thickness of the present shadow mask results in an unacceptably high proportion of the electrons in the electron beam hitting the side walls of the apertures in the shadow mask for low landing angles. This produces an unacceptable reduction of beam current impinging on the phosphor and a like decrease in picture brightness for low landing angles, e.g., landing angles less than about 25xc2x0.
One approach to this depth dilemma has been to seek a thin or so-called xe2x80x9cflat-panelxe2x80x9d display that avoids the large depth required by conventional CRTs. Flat panel displays, while desirable in that they would be thin enough to be hung on a wall, require very different technologies from conventional CRTs which are manufactured in very high volume at reasonable cost. Thus, flat panel displays are not available that offer the benefits of a CRT at a comparable cost. But a reduced-depth cathode ray tube as compared to a conventional CRT need not be so thin that it could be hung on a wall to overcome the disadvantage of the great depth of a conventional CRT.
Accordingly, there is a need for a cathode ray tube having a depth that is less than that of a conventional CRT having an equivalent screen-size.
To this end, the tube of the present invention comprises a tube envelope having a faceplate and a screen electrode on the faceplate adapted to be biased at a screen potential, a source of a beam of electrons directed toward the faceplate, wherein the source is adapted for magnetic deflection of the beam of electrons, and phosphorescent material disposed on the faceplate for producing light in response to the beam of electrons impinging thereon. At least first and second electrodes are interior the tube envelope, each defining a respective aperture through which the beam of electrons passes, wherein the first electrode is proximal the source and distal the faceplate and is adapted to be biased at a potential less than the screen potential, and wherein the second electrode is between the first electrode and the faceplate and is adapted to be biased at a potential not less than the screen potential. A third electrode is between the first electrode and the faceplate and is adapted to be biased at a potential not exceeding screen potential.
According to another aspect of the invention, a display comprises a tube envelope having a faceplate and a screen electrode on the faceplate biased at a screen potential, a source within the tube envelope of a beam of electrons directed toward the faceplate, a deflection yoke proximate the source of a beam of electrons for magnetically deflecting the beam of electrons, and a phosphorescent material disposed on the faceplate for producing light in response to the beam of electrons impinging thereon. At least first and second electrodes are within the tube envelope, each defining a respective aperture through which the deflected beam of electrons passes, wherein the first electrode is proximal the source and distal the faceplate and is biased at a first potential less than the screen potential, and wherein the second electrode is between the first electrode and the faceplate and is biased at a second potential not less than the screen potential. A third is between the second electrode and the faceplate and is biased at a potential not exceeding the screen potential. A source of potential provides the first, second, third and screen potentials.